Investigation into an improved breakdown voltage is now conducted for metal insulator semiconductor (MIS) type transistors such as metal oxide semiconductor (MOS) type transistors.
FIG. 15 is a cross sectional view of an n-type MOS transistor disclosed by J. C. Mitros et al. in IEEE transactions on electron devices vol. 48 pp 1751-1754 August 2001 ((a) of FIG. 1 in the transactions).
An n-type high concentration impurity region 102 of a drain is spaced apart from a gate electrode 100 by an offset length D. A low concentration n-well 101 is inclusive of the n-type high concentration impurity region 102, and extends to the region under the gate electrode 100 on the drain side. As a high voltage is applied to the drain, the n-well 101 is depleted in the region between the gate electrode 100 and n-type high concentration impurity region 102 of the drain so that a voltage applied to the substrate just under the gate electrode 100 at the drain side edge is relaxed. A drain breakdown voltage is improved in this manner.
MOS transistors having the similar structure that a high concentration impurity region of a drain is spaced apart from a gate electrode via a low concentration impurity region of the drain are disclosed in other documents, e.g., Japanese Patent Unexamined Publication 2005-093458, Japanese Patent Unexamined Publication 2006-319331, Japanese Patent Unexamined Publication 2005-136169 and Japanese Patent Unexamined Publication 2004-207498.
If a higher breakdown voltage is desired with the above-described structure, an offset length between the high concentration impurity region of the drain and the gate is elongated. However, as the offset length is elongated, a transistor on-resistance increases so that a transistor drive performance is degraded.